Recently, there has been an increased need for a method for generating electricity, the method taking the global environment into consideration. Technological development in microbial power generation has also been progressing. The term “microbial power generation” refers to a power generation method in which electrical energy is extracted, the electrical energy being obtained when microorganisms utilize organic matter.
Generally, microorganisms, organic matter utilized by the microorganisms, and electron carriers (electron mediator) are made to coexist in an anode chamber where an anode is disposed for microbial power generation. The electron mediator enters the microorganism, receives electrons generated by microbial oxidation of organic matter, and transfers the electrons to the anode. The anode is electrically connected to a cathode via an external resistor (load). The electrons which have been transferred to the anode move to the cathode via the external resistor (load), and then are transferred to an electron acceptor that contacts the cathode. Such electron transfer allows current to flow between the cathode and the anode.
For microbial power generation, since the electron mediator directly extracts electrons from a microorganism, theoretical energy conversion efficiency is high. However, actual energy conversion efficiency is low, and an improvement in power generation efficiency is sought. Therefore, in order to increase the power generation efficiency, various considerations and developments have been made with respect to electrode materials and structures, types of electron mediator, selection of microbial species, and the like (see, for example, Patent Documents 1 and 2).
Patent Document 1 describes that: a cathode chamber and an anode chamber are divided by an alkali-ionic conductor made of a solid electrolyte; the spaces inside of the cathode chamber and the anode chamber are filled with a phosphate buffer (a buffer) having pH 7; and air is blown into the phosphate buffer (a cathode solution) in the cathode chamber to generate power.
Patent Document 2 describes that: a porous body is installed as a cathode plate in such a manner as to cause the porous body to contact an electrolyte membrane which divides a cathode chamber from an anode chamber; air is distributed in the cathode chamber; and the air is made to contact a solution in a space of the porous body. Hereinafter, the cathode which utilizes oxygen in air as an electron acceptor while air is distributed in a cathode chamber in such a manner may be referred to as an “air cathode”.
The generator has advantages as follows: a microbial power generator utilizing the air cathode does not require a cathode solution; the generator also only requires to simply distribute air in the cathode chamber; and aeration of the cathode solution is not necessary.
Previously, in order to increase the power generation efficiency of a microbial power generator using the air cathode, the following were considered:    1) mediators for the anode (see, for example, Patent Document 3);    2) pH adjustment of the anode chamber;    3) types of cathode catalyst and methods for supporting the active ingredient of the catalyst; and    4) cathode structures.